This research program will develop a standard procedure for generating quantitative ultrastructural inventories of synaptic elements in given zones of cerebral cortex. This technique will then be used to study the effects of experimental manipulations which are known to, or which may produce, changes in cortical synaptic morphology. The procedure will be developed in experiments on turtle general cortex, the simplest cortex known to receive direct thalamic afferent projections. Selected regions of this cortex will be studied in serial 25 um zones from pial surface to ependyma or white matter. Populations of synaptic profiles will be categorized according to size of the presynaptic element, vesicle type, membrane differentiation, and the type of postsynaptic element. When the method has been shown to characterize a given cortical region within a reproducible range of variation, it will be applied in three sets of experiments. One set will analyze all of the connections of the simplest cortical synapse inventory by relating each component of the normal inventory to an experimentally identified cell source. These experiments will include a quantitative description of the nature and time course of structural changes in cortex following removal of known populations of afferent axon terminals. The second series of experiments is designed to compare regions of turtle cortex exhibiting different degrees of structural complexity. These comparisons will show the specific way in which neocortex-like systems qualitatively and quantitatively increase in synaptic complexity. In the third series of experiments, the quantitative inventory technique will be used to search for structural changes in turtle visual cortex which correlate with experimentally produced changes in visually guided behavior.